Use the fallthrough statement in switch loops

Clang does not seem to work with 'fall through' in comments.

Change-Id: Idcbe373be33ef7247548f856bfaba7ceb7f749b5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
diff --git a/src/lib/edid.c b/src/lib/edid.c
index 41b8031..06b9cee 100644
--- a/src/lib/edid.c
+++ b/src/lib/edid.c
@@ -1183,13 +1183,13 @@
 		switch (edid[0x13]) {
 		case 4:
 			c.claims_one_point_four = 1;
-			/* fall through */
+			__fallthrough;
 		case 3:
 			c.claims_one_point_three = 1;
-			/* fall through */
+			__fallthrough;
 		case 2:
 			c.claims_one_point_two = 1;
-			/* fall through */
+			__fallthrough;
 		default:
 			c.claims_one_point_oh = 1;
 		}